Mercer County is a county located in the U.S. state of New Jersey. Its county seat is Trenton, also the state capital, prompting the nickname The Capital County. Mercer County alone constitutes the Trenton–Princeton Metropolitan Statistical Area and is considered part of the New York Metropolitan Area by the United States Census Bureau, but also directly borders the Philadelphia metropolitan area and is included within the Federal Communications Commission's Philadelphia Designated (media) Market Area. As of the 2020 United States Census, Mercer County's population was 387,340, making it the state's 12th-most populous county, an increase of 20,827 (5.7%) enumerated at the 2010 U.S. Census, when its population was enumerated at 366,513, in turn an increase of 15,752 (4.5%) from the 350,761 counted at the 2000 Census, retaining its position as the 12th-most populous county in the state.
The county was formed by an act of the New Jersey Legislature on February 22, 1838, from portions of Burlington County (Nottingham Township, now Hamilton Township), Hunterdon County (Ewing Township, Lawrence Township, Trenton and portions of Hopewell Township), and Middlesex County (West Windsor Township and portions of East Windsor Township). The former Keith Line bisects the county and is the boundary between municipalities that previously had been separated into West Jersey and East Jersey.
Trenton–Mercer Airport, in Ewing Township, is a commercial and corporate aviation airport serving Mercer County and its surrounding vicinity. The official residence of the governor of New Jersey, known as Drumthwacket, is located in Princeton, and is listed on both the U.S. National Register of Historic Places and the New Jersey Register of Historic Places. Mercer County contains 12 municipalities, the fewest of any county in New Jersey, and equal to Hudson County. The county is located in the Central" target="_blank">https://en.wikipedia.org/wiki/Central_Jersey">Central Jersey region.